1. Field of the Invention
This invention relates to an ophthalmological measurement apparatus and more particularly to an ophthalmological measurement apparatus which projects a laser beam into the eye to be examined and outputs a specific measured value based on the state of scattering of the laser beam within the eye.
2. Description of the Prior Art
Measurement of protein concentration within the anterior chamber of the eye is highly important in determining the presence of inflammation within the eye, namely whether or not a blood-aqueous barrier functions normally. Conventionally, the general practice has been to use a slit lamp microscope and to make the determination by visual observation on the basis of grading. While a quantitative method based on photographic measurement has been reported, the fact remains that no easy method for clinical application has yet been developed.
Where judgment is based on visual observation, the judgment standards will differ from one person to another, degrading the credibility of the data obtained. For overcoming this problem, there is used a method of ophthalmological measurement in which a laser beam is directed into the eye and the light scattered within the eye is quantitatively analyzed (see U.S. Pat. Nos. 4,711,542 and 4,832,043, for example).
Where a laser beam is used for ophthalmological measurement, however, the intensity of the scattered light is extremely weak. The results of the measurement are therefore easily affected by noise components, i.e. by any light present other than the light to be measured. For example, if the measurement is carried out with respect to the anterior chamber and the point of measurement is too close to the crystalline lens, light scattered by the lens will constitute noise and the results of the measurement will vary depending on the location.
Moreover, the cornea has a strong lens effect and any light other than that impinging normally thereon is refracted thereby. This means that the amount of refraction varies with the point of impingement, with the result that the relationship between the point of measurement (the convergence point of the laser beam) and the point where the light is received (the mask) deviates. As the depth of the humor aqueous in the anterior chamber is about 3 mm, it is necessary to focus the laser beam at an intermediate portion lying at a depth of between 1 and 2 mm and to receive the light scattered from this measurement point with high accuracy. This requires accurate alignment between the apparatus and the eye under examination, particularly in the horizontal direction, and also makes it necessary to have a method for confirming that the required state of alignment has been achieved.
The present invention was accomplished in view of these circumstances and its object is to provide an ophthalmological measurement apparatus which enables alignment between the eye under examination and the apparatus to be obtained with ease.